rust: add support for imported functions

[axic]

[codecov[bot]]

Codecov Report

Merging #782 (8763a7f) into master (2bd08ab) will decrease coverage by 0.70%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
- Coverage   99.27%   98.57%   -0.71%     
==========================================
  Files          88        2      -86     
  Lines       13296     1053   -12243     
==========================================
- Hits        13200     1038   -12162     
+ Misses         96       15      -81     

Flag	Coverage Δ
rust	98.57% <100.00%> (+0.09%)	:arrow_up:
spectests	?
unittests	?
unittests-32	?

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Impacted Files	Coverage Δ
bindings/rust/src/lib.rs	98.84% <100.00%> (+0.07%)	:arrow_up:
lib/fizzy/capi.cpp
lib/fizzy/instantiate.cpp
test/utils/hex.hpp
lib/fizzy/asserts.cpp
test/unittests/utf8_test.cpp
test/unittests/execute_death_test.cpp
test/unittests/typed_value_test.cpp
test/utils/hex.cpp
test/unittests/parser_expr_test.cpp
... and 77 more